Perilla mint is an erect annual plant of the mint family. It was introduced as an ornamental plant because of its attractive green leaves with purple leaf edges, but escaped cultivation and is now a serious poisoning threat to horses and other livestock.

Perilla mint grows in moist soils of pastures, woodlands, stream banks, and other fields in most of the eastern United States. It can reach two feet in height at maturity. Leaves are opposite and coarsely serrated, and the stems are branched, square, and frequently have a purplish color. The plant has a strong pungent minty odor when crushed.

Perilla mint plants are toxic to horses, and all plant parts (especially the flowers and fruits) contain the toxin. The greatest risk is consumption of fresh plant material during late summer or early fall. Perilla mint plants harvested with hay also pose a risk to animals ingesting the dried plants. Animals that have consumed perilla mint might exhibit clinical signs of respiratory distress, such as labored breathing and possibly an elevated temperature.

Look for perilla mint in spring and early summer growing in moist, shady pasture margins. Mowing low to the ground in late spring is usually an effective control method. Consult your local Cooperative Extension Service personnel for herbicidal control in your area.